As a study on mechanism of photodynamic action with methylene-blue, the authors examined structural correlation of 40 kinds of nucleic acid-bases, nucleosides and related compounds with their photodynamic degradation by visible light-irradiation in the presence of methylene-bule. As a result, it became established that : 1) Above-mentioned compounds, in order to be easily photodynamically degradated, need to have the lactim structure with respect to N 1 and 3-position in pyrimidine moiety of purine ring. 2) Imidazol ring seems to be essential for photodegradation. 3) Existence of substituent in 8, 9-position in purine didn't affect on photochemical reaction.